Hey,
thanks for your message here. As I’m only a “mod”, I only can ping @bw-admin to ask for looking into this and what may have gone wrong with your account now (and to “reset” that, if possible).
Because, when you wanted to reply in the thread we discussed your issue (Best practice to use Bitwarden as passkey provider), I can’t see a reason why you shouldn’t be able to post there again. (to quote you: “I was informed that the thread had exceeded a weirdly artbitrary limit of 18 posts/replies”)
(there are some restrictions in general, but I would think they shouldn’t apply here, as you didn’t post the third time in a row, and there was also some delay between our posts 
)
[Edit to the text I just “crossed out”: don’t listen to me - listen to @DenBesten’s and @grb’s correct replies here down below!]
Whether the restrictions, when you login with GitHub to the forum, are “a bug or a feature”, I don’t know. Maybe @bw-admin can also say more to that.
Apologies from my side - I didn’t think of not logging in here the traditional way 
(password…) and I didn’t consider, if it would work to create passkeys, if one uses “social login” to the forum…
One person is only allowed to have one forum account, but I would say, though a bit hesitant and only for testing purposes, temporarily creating another forum account (with a password) and deleting it afterwards might be okay (it is for using Bitwarden…). – Or you think of and choose another site, where you can further test to create a passkey…
I hope I tackled everything I could for now.