Barring a very minute number of exceptions, 2026 will be the absolute last of the 7th year seniors, since they will have been the last class who could take advantage of the COVID exemption.
There may be a few remaining, though, such as those who entered the military and returned to college to play football. During your military service time, the eligibility clock pauses for as long as you're in active service.
For everyone else, the class of 2020 will be the last of the ones who could take the COVID exemption. These three redshirt years you could take include:
1) Conventional redshirt (play in 4 or fewer regular season games)
2) Medical redshirt (injured early in the season, lost for the year)
3) COVID exemption in 2020
Quite a few guys from 2020 took that year as a COVID exemption, since it was a "use it or lose it" situation. This way, they preserved the conventional redshirt year.
Every 7th year senior in 2026 will have used a medical redshirt year as well, and the NFL scouts tend to frown upon seeing a player who took such a redshirt year, since it had to be a significant injury.