The Bitwarden account itself needs to have limitations on what can be typed out on a keyboard, and I can fully understand the limitations there. It is perfectly reasonable to have those entries limited to the one-byte and possibly two-byte UTF-8 character set.
My concern arises with other passwords for which length is a forced limitation but character set is not. I know of several sites that limit passwords to 20 or even 16 characters, yet permit anything up to and including 4-byte characters (I can enter the full number of characters even if some are strictly 4-byte).
I would like to propose a fifth (at minimum) checkbox that can extend the generation to 2-byte characters and above.
As well, this functionality - to protect against confusion - can be protected behind a “power user” feature flag, such that it is hidden from anyone who has not turned on that toggle. I would recommend multiple checkboxes for two-byte, three-byte and four-byte characters, allowing people to populate their passwords with increasing complexity. Having a “limit x to y% of password” would also be helpful, especially on the desktop version (I manually create almost all passwords there).
Staff: I am a C# dev. I have no problem creating a mock-up if I am provided access to a sacrificial branch to work with that has the ability to launch the generator by itself.