I'm not seeing this difference of guideline versus requirement between the editions.
The basic roles have not really changed between the editions, 4e is just explicit about labeling some roles.
In 3e if you don't have a cleric or druid as a healer then you have a paladin, ranger, bard, or UMD rogue with wands of cure light wounds with only minor healing in combat or you use potions and pull back after fights to heal up.
I've played in a group where as a ranger with a wand I was the only healer for multiple levels of play and it went fine.
I wouldn't say that anything other than a pure Divine caster works anywhere near as well. Sure, other classes can almost make up for the healing difference (though they never get the really good healing spells), but something like a UMD Rogue can never make up for the lack of a Cleric's ability to use status restoration effects like Remove Disease, Restoration, or the extremely valuable Raise Dead and Resurrection. Even a secondary healer like a Bard can't remotely make up for the loss of a Cleric when it comes to condition restoration. If the PCs are fighting things that inflict curses, use poison, spread disease, and inflict ability score damage, which are all pretty common in 3E, then a Cleric is necessary.
In 4e if you don't have a leader you are limited to second wind in combat and then have to survive until out of combat. I like surges as they make wands unnecessary. They are an improvement in not needing a healer.
See? You are agreeing with me here. 4E has elements that make a dedicated healer a bit less necessary. Of course, the fact that 4E makes all the condition restoration spells which force a team to have a Cleric into Rituals also helps a lot. Now, if you have a party member who is suffering from a disease, any party member with the Ritual Casting feat can help.
In 3e if you don't have a trapfinding rogue you use certain magics to get around traps (fly over pits, neutralize poison to get around poison, heal the damage taken, clerical find traps will not help you) or you take the effects of the traps and soldier on (the barbarian trap finder method).
In 4e if you don't have a rogue or someone with thievery do you have more options than in 3e?
You are equating things that are not remotely the same. 3E requires a Rogue. 4E requires a character trained in the Thievery skill. Those are very different things. Sure, it may be a bit harder to deal with traps in 4E if you don't have a character with the Thievery skill, but there are several classes that have it as a class skill, anyone can get trained in it at the cost of only one feat, and all uses of the Thievery skill can be done untrained barring DM fiat. Compare this to the following quote from the 3.5E SRD: "Rogues (and only rogues) can use the Search skill to find traps..". If you want to find and disarm traps in 3E, you need a Rogue, but literally anyone in 4E can do the same with just a few skills.
This brings up a point regarding Clerics and Leaders. Sure, a 4E team needs a Leader about as much as a 3E team needs a Cleric (it may not be strictly necessary, but it really, really helps), but a Cleric and a Leader are not equivalent to each other. In 3E, you really need either a Cleric or one of the few other full Divine casting classes (which all strongly resemble the Cleric), but in 4E you only need a class that fills the Leader Role, which is a broad category of classes of which many are very different from the Cleric. In 3E, the more a class with healing magic is different from the Cleric, the less effective of a healer it is, but all 4E Leaders have a solid baseline of healing capability.
I honestly don't care whether you call these things requirements or guidelines, but there is a massive difference in how limiting they are. 3E requires or strongly recommends specific classes, and 4E only really asks that you have one character's class be chosen from a reasonably large list and that at least one character has the Ritual Casting ability. That is the central point.