The five substitutes rule was introduced during the COVID-19 impacted seasons to help teams manage their squads on the understanding that many may be impacted by the disease in some way and to allow the flexibility to better manage player wellbeing.
I was surprised this arrangement was taken on in Scottish football as a permanent change.
I believe it confers a big advantage to the wealthier clubs with the bigger squads.
In particular Celtic with close to 30 first team squad members can now draw on a bench of international players each of whom can come on without greatly weakening the team.
Given the style of Ange Postecoglou’s football, the high intensity pressing off the ball, the constant movements and rotations in attack, it is a physically demanding style of play. Therefore, being able to change up half the outfield players is a real bonus.
Celtic also dominate possession with 70-75% possession being normal against SPFL clubs.
That means the opposition players inevitably start to tire around the 60–65-minute mark.
Whilst those clubs cannot bring on like for like talent due to the resource constraints much smaller clubs operate under, Celtic’s starters can give everything for 60 minutes and then effectively replace the whole front five attacking players with equally effective subs.
Celtic rarely swap out any of the back four or Callum McGregor. The front five (i.e. the two number eights, the two wingers and the striker) are usually swapped out from 55 minutes out.
League Impact
Here is the goal difference in the league in the last 30 minutes of games:

This is a pretty strong correlation to squad budget.
So, the five-sub rule exacerbates existing inequalities.
Player Impact
How does it impact individual player performances?
Here is the expected scoring contribution (xG + xA) per 90 minutes for all the attacking players as starters and as subs.

Only Aaron Mooy and James Forrest, who are coincidently the oldest outfield players, average higher xSC when starting versus when coming on as subs. This may be due to their relative declining dynamism.
Liel Abada pretty much does not care whether he starts or not, he’s busy.
But every other player has a higher average xSC coming on as a substitute than when they start.
This is logical. Many SPFL matches, as the opposition tires and cannot replace their starters with stronger players off the bench., so space and time open up. If Celtic are already ahead then some stat padding can he had! That is, relatively cheap extra goals and assists.
The average across all the attackers is that your xSC will be 0.36 higher if you come off the bench that if you start. A significant difference.
The five-sub rule has really helped an already strong Celtic squad.
Clearly the SFA and SPFL did not anticipate that!