It turns out, that only keys that are shown (in the “passkey”-2FA section of the web vault) as “Migrated from FIDO” are being phased out. But even those should work after re-registering them (and by that, they would be automatically registered with the newer WebAuthn protocol):
(from: Release Notes 2024.12.0)
Here, also some info from Micah from Bitwarden: FIDO U2F keys are being phased out in 2025 - make sure to replace those in time - #38 by Micah_Edelblut
→ so in the end, only the older U2F protocol get’s phased out - and not older security keys in general… as even older security keys, when now (re-)registered, get automatically registered with the newer WebAuthn protocol and continue to work then…