There are http://www.commandlinefu.com and the like. I copy and paste stuff from the web into the shell occasionally, I'm sure everyone does. I do use fish, but it is annoying sometimes, it's missing the most basic stuff like backticks. So much so that I think I'm going to stop.

I'm sure you know this, but it's not missing that feature. It's just that the syntax is different.

For starters, any utilities based on sourcing won't work, such as nvm/rbenv/etc.

That's technically true for zsh in cases, but it's far easier to port or add compatibility for zsh than fish.

You can use bass which solves this problem without porting the script to fish: https://github.com/edc/bass