I would hazard a guess that the number of real and important situations where a critical and irreplaceable legacy system requires access to *Gmail* is close to zero. These systems are unlikely to have access to the internet, and if you’re connecting an *unsupported browser* to a legacy webmail solution, you’re doing things very, very wrong.

What are you thinking of self-hosting? The email server? There are plenty of alternatives to GMail. An email webclient? Just use a non-web client, there are plenty of those too :-).

For a webmail client probably https://squirrelmail.org. It's over 2 decades old, still maintained, and kept to its original simplicity.