Is there a FOSS alternative to apps like TickTick?

I read that title as "Is there a FOSS alternative to apps like TikTok?"

tasks.org is pretty excellent

https://alternativeto.net/software/ticktick/?license=opensource

Super Productivity has some integration limitations, but generally been working great for me.

Try Vikunja. Project Segfault has an instance, but you can host your own if you want.

Significantly depends on your needs. I need a full functional web client, what makes list of free alternatives basically empty.