Not to mention legacy code. :D

jQuery still has its uses. There are better more efficient ways of achieving results, but it all depends on the ecosystem you're developing in/for. Focus on JS, and the jQ API docs will help you out where/when/if you need to work on something in jQ.

I don't think there can be such a thing as overkill where any client data is concerned in this day and age. If you're not using PII, then Mohamed Binothman's suggestion for JWT is reasonable.