Is there a Linux equivalent to Windows alt codes?

I'd try a Compose key. You assign an otherwise unused key to Compose, then type a short sequence to make a character. For example, Compose, apostrophe, e produces é, and Compose, 1, 2 produces ½.

It's a different habit from numeric alt codes, but many of the combinations are easy to remember. Look for Compose key options in your desktop's keyboard settings.

Check your keyboard layout too. Many layouts have extra characters on layers accessed with AltGr, often the right Alt key. An international layout may make common accents easier to type without memorizing codes. The exact combinations depend on the layout you choose.

For an occasional character, Unicode input is another option. In many Linux applications, press Ctrl+Shift+U, type the character's hexadecimal Unicode value, then press Enter. For example, e9 gives é. Application support can vary, so try it in the editor you actually use.

If you have a lot of alt codes memorized, there are scripts intended to reproduce that workflow on Linux. That could save some relearning, but check whether one supports your desktop session and the applications you need before relying on it.