Your speed is halved, after subtracting for encumbrance. (It'd be too brutal to halve it before the subtraction.) For the purposes of the speed checks in chases and for seeing how quickly you can move laterally. (I.e. it probably takes you two turns.)

Everytime you use the Dash action you fall prone afterwards.

You have disad on balance-related ability checks.

You need Regenerate spell to grow back the leg.

TBH I have an IRL close friend who is in this sitch so I don't want to make light of it. Trying to fight ablism and ablist jokes.

These are the rules from DMG p 272. They are for the type of prosthetics you can make in the D&D world. As for my friend, I couldn't even tell before she told me. I wouldn't have guessed. She's in pain walking but it looked normal with clothes, socks and shoes over. Not that I saw her run or anything but we went on walks and she walked normal speed. But she didn't have a monk halfling carve it in the jungle either.♥