The problem with the current rule and the suggested rule:
All it really does is slow down the game pace. I know the intention of the rule is to give the team being rolled a chance to coordinate a comeback. However, the current ruling is hard to enforce and falls into the gray area quite a lot. For example, the debate about strapping C4 to a vehicle and blowing it up right after it leaves the spawn. That is the exact same thing as killing them in spawn, just with a bit of delay. As soon as they entered that vehicle they are set to die.
The suggested rule just makes the existing problem worse. I heard someone in vent mention it makes the game passive which is exactly what it does. Now you have two possible scenarios for in-game when a team is getting rolled. Either one team will just camp right outside the spawn and wait for them to come out or shoot them as soon as they shoot (squad spawning makes this all too viable). The other instance would be you just wont be able to restrain a whole team of pubs from doing their own thing and charging that spawn, it's hard enough to prevent them as it is. People join the game to play, if one person is saying its the rule that you can't enter the enemy spawn the pub will just leave to find a server where they can shoot something. Those who don't leave willingly are going to run in and you'll have to kick them anyway. The servers will just end up empty whenever a team rolls another team under the proposed rule. The current ruling also causes people to leave, but more slowly.
A different idea: Make the spawn camping rule situational.
Remember the intention of a "no spawn camping" rule is to allow a team a fighting chance. However, I think we would all agree at a certain point, there is no way a team can make a come back if the ticket difference is too great. Therefore, say a team is getting suppressed in their spawn, the spawn camping rule should depend on the difference of tickets between the two teams.
For example:
The match is set at 250 tickets per team. One team is getting rolled early on, the ticket count quickly changes to 225 vs 200 (25 ticket difference). At this point the current rule (or the proposed rule) for spawn camping should be enforced. However, later in the game say the tickets have now changed to 200 vs 100 (100 ticket difference). At this point the chances a team can come back is much lower, all the spawn camping rule does now is delay the end of the match and the start of a fresh round. Therefore I say once the teams pass that threshold where the team getting rolled has little chance of making a comeback to simply remove any spawn camping rule. The values I used in this example is just for the sake of the example, if this rule garners support the difference in ticket value would be subjective to the server settings.