In France we treat our prisoners like utter shit. If they way you treat the people you have power over is an important marker of civilization/democracy (and I believe it is), we fail this test real hard.

That being said, the tribunal has to specifically add to the prison sentence an exclusion from the right to vote. Iirc, about 25k prisoners (among the 75 or 80k total) have been deprived from the right to vote during their sentence.

Voting from prison in France is complicated,there are 3 options afaik:

• you can delegate your vote to someone on the outside
• you can resquest a "day off" to go to the polls
• since 2019 you can vote by correspondance

The "can I please go out to vote" has to be approved by the warden, and dosen't happen much.
Delegating your vote isn't always easy either, prison has a tendancy to isolate people from their former close ones.
The correspondance vote is recent and seems like the best of the three. In 2017 (presidential electio ), less than 2% of imprisoned people had voted. In the 2022 presidential election, more than 20% of them did.

So far, voting logistics and the feeling that society doesn't want you has imo prevented far more people to vote than the "you can't vote for the next x years" addendum to sentences.