By what legal mechanism was Catherine Howard deprived of her title of Queen of England?

It is not clear that a deprivation of title actually took place. There was a formal reduction in Katherine's dignity when she was transferred to house imprisonment in Syon Abbey, but official papers continue to use the word "Queen" up to her execution. That includes documents from the Privy Council, the courts dealing with treason allegations against people in her circle, and the act of attainder itself. We do not appear to have any formal instruction regarding the title, from the Council - including in its capacity as the Court of Star-Chamber - or Parliament, or the King. There is nothing comparable to the Act of Parliament of 1540 (not long before!) which deprived Anne of Cleeves of the same title.

Looking at some of the source documents from this time, it could be that what is happening is this:

1. Following initial investigation into the allegations against Katherine at the beginning of November, there is a doubt about her "pre-contract". One of the rumours was that she had promised marriage to Francis Dereham prior to a sexual relationship with him, all before marrying the King. One legal theory (according to a certain view of canon law) was that this might make her married to Dereham, in which case she was not married to Henry, and as a matter of simple fact would not be Queen. There was thus a prudent doubt over whether she was Queen at all, although this line of inquiry was not long pursued, being eclipsed by the Culpeper charges of adultery after her marriage.
2. On 11 November, Katherine was removed to Syon. She was permitted a limited number of servants and had various courtly luxuries removed. The Archbishop of Canterbury, Thomas Cranmer, was the official charged with these arrangements by the Privy Council. The letter of Council to Cranmer narrates that Katherine is no longer worthy of living as a queen, although she remains factually queen for the time being.
3. The French ambassador, Charles de Marillac, writes to his King on 22 November, describing the events above. He says that Katherine's ladies-in-waiting have been told not to call her "queen".
4. Skip forward to the nineteenth century and the publication of Henrician state papers. An English version of Marillac's letter says that she was "henceforth to be named no longer Queen", without the detail that the final instruction was only to Katherine's household.
5. Relying on the accessible English summary, various English-language publications assume that Katherine was stripped of the title for all purposes, on the 22nd or thereabouts.

Working backwards, here is the English version of Marillac's letter:

As soon as the case was proved, proclamation was made at Hampton Court that she had forfeited her honour, and should be proceeded against by law, and was henceforth to be named no longer Queen, but only Katharine de Auvart.

This modern precis is in Letters and Papers, Foreign and Domestic, Henry VIII, Volume 16, 1540-1541, eds. James Gairdner and R. H. Brodie (London: HMSO, 1898), entry 1366. The original text is cited to Correspondance politique de MM. de Castillon et de Marillac, ambassadeurs de France en Angleterre (1537-1542), Jean Kaulek (Paris: Alcan, 1885), entry 376 at p366 where the key sentences appear as:

[A]près que le cas esté advéré, l'on a faict incontinent publier à Hantempcourt où elle estoit entre les dames qui luy tenoient compagnye comme ce roy trouvant qu'elle avoyt forfaict de son honneur avant qu'il la print et après aussy, par advis de son conseil avoit résolu de ne la tenir plus à femme ains de procedder contre elle ainsi que les loix en ordoinnent. Et par mesme moyen fut enjoinct ausdites dames de se retirer chacune de sa maison et doresnavant de ne la nommer en aulcune sorte royne, ains seullement Catherine de Auvart.

In this full version, we see the distinction that the royal title instruction is limited to the ladies accompanying Katherine. It is part of the way in which she was removed from courtly life; the King is no longer treating her as his wife, but mounting a formal legal process against her.

This letter is dated 22 November but refers to events of weeks earlier; Katherine had been removed to Syon on the 11th, which is also when the allegations of her case were made public. We have the letter sent by the Council to Thomas Cranmer, instructing him to convey Katherine to Syon and describing related plans - item 163 in State Papers of King Henry VIII (London: HMSPO, 1830). That says with equal nuance:

Considering no man would think reasonable, that the Kings Highnes, although His Majesty doth not yet take the degree of her estate utterly from her, should entertain her so tenderly, in the high degree and astate of a Queen, who, for her demerites, is so unworthy the same [...]

This instruction of Council is consistent with the reading that Katherine was being placed in a sort of limbo, where her royal dignity was being suspended while the legal process unfolded. Her marriage to Henry remained legally intact up to her execution. The various proceedings in courts and Parliament do not hesitate to call her queen throughout this period. In the whole matter, Henry appears to have taken effort to manage public opinion, by strategic publication of details of the matter, and gathering indictments from across the country - he wants to be seen as following a proper course of action. What we do not see is a royal proclamation forbidding use of the title (the kind of thing we surely would see if the King wanted people not to use it - they have to be told!).

For other members of Henry's family, there were other struggles with titles.

• Katherine of Aragon continued to use "queen" for herself, despite repeated orders that she should be "Princess Dowager of Wales".
• During Anne Boleyn's tenure as queen, several Acts were passed making it treason to question her status.
• Anne of Cleeves was commonly called "princess" or "Lady Anne" after the end of her marriage; Henry also wrote declarations saying that he would consider her "sister and servaunt". By Act of Parliament (32 Hen 8 c.25), Anne "shall not be named or called within this Realme nor in any other the Kings dominions nor ellis where the Kinges wife nor Quene of this Realme".
• Henry's daughter Mary was called "Lady Mary" rather than "Princess" for many years, due to the legitimacy problem.
• His other daughter Elizabeth also had a period of being considered illegitimate, losing the title Princess Royal.

These other examples illustrate some practical difficulties with enforcing use or non-use of a controversial title. In particular, they show the use of Parliamentary authority to reinforce or deny a claim. This was not done in the case of Katherine Howard - perhaps because her execution would make it a moot point.