I'm not on that specific tracker but some of the others have similar sort of rules

They have something about changing IPs, it seems you can’t change IPs too often and you can’t have multiple accounts in the same IP. Why?

That's generally for detecting ban evasion / duplicate accounts. But I think with most private trackers you'd just need to let staff know if you're in a shared IP situation.

They absolutely not allow mobile client, which means, you need to let your PC on all the time, sum this up with the fact that they require 72h seed within the next 30 days, this is a t least 2.5h of seed daily and if you miss one day for any reason it’d be even more for the next days. But my question is about the mobile, why the hell can’t I use my phone to seed? I talked to a mod and they argue mobile isn’t reliable due to battery optimization and it might kill the client. What does this even mean?

The mod is sort of right about that one. Most mobile users aren't even connectable (port forwarded) so that's problem number 1. And problem 2, yes Android does pause apps for battery optimization - but technically you can configure your Android to not do that for specific apps, not sure if it works for all phones but in mine I just bring up the App Info, go to App Battery Usage, and make sure to enable "Allow Background Usage".. most people wouldn't really want to do that and run down their phone battery constantly but I guess it's an option. Maybe bring that up as a tracker suggestion/rule change though it does seem easier just to tell members not to use mobile clients.

Why the torrent clients are so limited even the version you can use? What it has to do with anything at all? Again, is not seeding just enough?

Maybe specific to that tracker, not sure. But usually torrent client lists exist because the tracker staff tested those clients against the private tracker software to verify that stats are being tracked correctly. And sometimes they do ban clients due to being buggy, being used for ratio cheating, that sort of thing.