Title. I have only ever played season in the past and went to play some old chars tonight. I cant play adventure mode until I finish the full campaign?? It also looks like I lost quite a few items on my chars, is this normal?

N never heard about lost item. No way blizz deleted it.
Concerning adventure yes, you need to finish campaign.
Fastest way is to ask someone in chat to invite you in last quest and do it (rush and skip useless story part is recurrent in diablo games)

If anyone is willing to invite me to the last Quest that would be awesome. Wouldnt mind tipping of course.

Your best bet is to post in general chat in-game, you will have it done in seconds

If you were #1 nec like me you would have helped him and he would have been done in seconds.

Git gud scrub

Sorry my bestest necro only PvP

The lost items likely timed out. After the season ends and the toons get moved to non-seasonal all of your items go to the gift post (the little thing that looks like a mailbox near the stash). There should be an envelope with a green leaf. The items will stay in that stash for 30 days IIRC. After 30 days, the items are deleted. So if it has been a while since you played the "old" char, there is a good chance the items just expired in the seasonal stash

Sounds more like a shako nec imo

Nn hat, just skillz