The traditional rules are, that you have up to 5 years to finish out 4 years of eligibility. This is assuming that you take a standard redshirt year, where you don't play more than 4 regular season games.
You're only allowed a single standard redshirt year, and beyond that year, it doesn't matter if you play or not, you burn up a whole year with each year of time passing.
You can take a medical redshirt, where you shut it down for the rest of the year after sustaining a severe injury early in the season. This can grant you the rare 6th year. Without the COVID exemption in place, 6th year seniors were a very rare thing indeed. I remember one guy from the Miami Hurricanes, Rusty Madearas, who was granted such a thing after all but shredding his knee, and missed two seasons because of it.
The COVID exemption policy was granted for the 2020 season, where you could play the whole year, and it wouldn't count against your eligibility. If you didn't play enough games for it to count as a full year, you couldn't take advantage of this.
Pyne's case was like this:
2020 - Played in 4 games, but the year didn't count, due to the COVID exemption. This way, he didn't have to burn through the standard redshirt year.
2021 - Played in 2 games, took a standard redshirt year.
2022 - Played pretty much a full season. 1st year of eligibility used.
2023 - Played in 2 games at Arizona State, got injured, in addition to aggravating his pre-existing knee injury, along with his bad hamstring. Granted a medical redshirt.
2024 - Played in 6 games at Missouri. 2nd year of eligibility used.
2025 - Played in 1 game so far at BGSU. Regardless of whether he plays any more or not, it counts a his 3rd year of eligibility used.
2026 - Regardless of what happens, this will be his final year of eligibility, and he will not have any more left. He can't even play at a FCS school.
He's in the same boat as Cade McNamara, but this year is McNamara's last season, no matter what happens, since McNamara was in the class of 2019, Pyne was in the class of 2020.
Starting in 2027, it will be mathematically impossible for someone to have a COVID exemption, unless there's some really far-out, whacky circumstance, such as deciding to do military service for that 2026 year, and coming back after 4+ years of service, since he'd still have that final year of eligibility remaining even if he's over 30 years old at that time.
It's all but impossible for that to happen, since I'm pretty sure he's not destined for military service.