The last two days my friend and I have been trying to finish off our Lilith altars to be ready for season 1 and I think everyone is doing the same. Every time I roll up to an altar at least one person but usually two or three people come charging in on their horse. It's fun to be here at the start of this game's journey and seeing everybody getting ready! I think I'm gonna try Druid when the season starts 🐻
If you make a new character to be a part of the season, yes all side quest will be reset as you are playing a different character. Only campaign completion, alters and map explore will carry over to a new character.
Nothing happens to your current character though, nothing is deleted and nothing is reset. You can continue on that character forever, you just will not be a part of the season.
You can ignore them if you want, or grab them while you do the story in the season if you want. I've leveled a couple of alts to the nightmare dungeon stage, and with some of them, I haven't touched a single side quest. I do world events and dungeons to level. On other alts, I'll do side quests if I happen to see one, or I'll pick it up from the quest giver and if I happen to be near the objective, I'll do it.
Seasona reset everything every time they start (except this first one where they're keeping some of the world stuff). You can do the reset stuff, or not. It's up to you. But I wouldn't quit over it because it's not really required.
They were originally going to have everything be a clean slate, as I would typically expect for a seasonal start. Reportedly, they changed due to "overwhelming community feedback" so that's why we have this awkward "keep the map visibility and altars" solution.
I'd rather stab out my eyeballs with a fork than having to do altars again tbh. One of the most tedious things I ever had the displeasure to do in gaming.