Is it possible to use S/MIME in the web client of Gmail?

There is Penango - https://www.penango.com/products - which seems to have a free edition for GMail. I looked into it a while back, since as you say, there isn't anything all that up-to-date, but haven't tried it (ended up going with S/MIME in Outlook checking my GMail account).

Another option is Fossa Guard - S/MIME extension for Google Chrome browser. S/MIME certificates are free. And it is interoperable with many other mail clients.

Use R2Mail2 on Android to read, decrypt, verify, sign and encrypt emails of both PGP, PGP/MIME and S/MIME formats. You can use the IMAP+SMTP interface to Gmail.