Note that you can store longer state (at least 64K; more not tested) in the fragment (`location.hash`); obviously only the client gets to see this, but it's better than nothing (and JS can send it to the server if really needed).

For parameters the server does need to see, remember that params need not be &-separated kv pairs, it can be arbitrary text. Keys can usually be eliminated and just hard-coded in the web page; this may make short params longer but likely makes long ones shorter.

You absolutely should not restore state based on LocalStorage; that breaks the whole advantage of doing this properly! If I wanted previous state, I would've navigated my history thereto. I hope this isn't as bad as sites that break with multiple open tabs at least ...