[2020] Can't place plazas and some parks outside of "Park Districts"

Parks should be fine to be placed outside park areas - they just need road access when that's done. If you can't do that, then it sounds like a mod is doing something it shouldn't, and we'd need a full list of your mods to help.

Those are both regular parks, so definitely a mod problem.

The easiest way for me to have a look through your mods would be for you to post your output_log.txt (called player.log on Mac and Linux).

Here's where to find the log depending on your operating system. To easily find the local files for the game, right click your game in the Steam library, select Properties, go to the Local Files tab and select Browse local files.

Windows
\SteamApps\common\Cities_Skylines\Cities_Data\output_log.txt

Mac OS X
Users//Library/Logs/Unity/Player.log

Linux
~/.config/unity3d/Colossal Order/Cities: Skylines/Player.log

As we can't attach files here on Steam, you will need to upload the file somewhere else. Here's a few sites you can upload it to, which do not require you to sign up first:

https://uploadfiles.io/ - free, no sign-up, can handle large log files
https://paste.ee/ - fast, no sign-up required (some log files will be too large for it though)
https://zerobin.net/ - bit clunky, but can handle huge files, no sign-up required