IOS High Sierra vs IOS Catalina

I have IOS High Sierra on my MacBook Air and know that I need to upgrade the IOS; however, I see that so many people have complained about IOS Catalina. I have no issue with my IOS High Sierra. What should I do? If I don't update the IOS, can I still upgrade it to a better IOS for free later on?

Another question is my MacBook Air does not sync the reminder from my phone and iPad Pro 2020 even though I set Reminder on sync already. What should I do?