https://blog.chromium.org/2021/05/update-on-user-agent-string-reduction.html
Is helpful to read. Some key points:
"Beginning in M92, we plan to start sending deprecation notices for the navigator.userAgent,
navigator.appVersion, and navigator.platform getters in the DevTools Issues tab."
"If your site, service, library or application relies on certain bits of information being present in the User Agent string such as Chrome minor version, OS version number, or Android device model, you will need to begin the migration to use the User Agent Client Hints API instead."
I know I am not using the navigator getters in question so at this point, it seems I can only wait for an update to the library's .js
(in my case, bootstrap 4) to make the warning go away.