Dungeon difficulty has slowly gone up in terms of mechanics, but the overall difficulty has gone down based on the maximum survival threshold of our job's toolkit went up in a party setting. Look no further than how tanks improved from Stormblood's dungeons in a 4-man / Shadowbringers' dungeons during Shadowbringers expansion and Endwalker dungeons in a 4-man currently.
The dungeons in Stormblood actually required a healer to power through it because tanks didn't have natural self sustain. The amount of sustain they had was a lot lower that they can die to bosses from unavoidable damage.
In Shadowbringers expansion, tanks have gotten better mitigation. PLD and WAR in particular went crazy strong in terms of self-sustain when push comes to shove. WAR can now use Nascent Flash (HP recovery per enemies hit) over Raw Intuition (flat damage reduction). PLD being able to keep one to 2 other players alive during Malikah's Well Final Boss even if a healer is dead purely from the lowered MP cost of Clemency being able to keep up with their MP generation (4000 MP -> 2000 MP) and Atonement. That being said, tanks still mostly relied on healers. Healers were still pretty important when things go south, but the amount of healing tank can start being able to cover gave a LOT more room for error in terms of finishing a duty - thereby drastically reducing the difficulty of the dungeon.
In Endwalker, every tank has the ability to self-sustain and can do dungeons without bringing a healer (which only speaks volumes to how much extra room for error that healers -- whose entire toolkit is basically healing now-- can provide to prevent a wipe when in the past, they were a necessary component). Parties even sub in a third DPS if they are premade for that reason because the innate self sustain of a tank is enough to offset the total amount of unavoidable damage in most cases. WAR had their Raw Intuition reworked so it will now recover HP in dungeons starting from lv 56 onwards rather than wait until they get Nascent Nascent Flash at lv 76. PLD gained healing on both Sheltron (Now Holy Sheltron applying both mitigation + regen effect) and in 6.3, PLD got another rework where they have healing attached to every Divine Might combo/Requiescat usage.
A more extreme and easier to see example of Dungeon's mechanical difficulty versus overall difficulty would be the Lv 47-49 dungeon Aurum Vale. If you compare Endwalker dungeons and then you look at dungeons like Aurum Vale, you can see the difference easily. You can say Aurum Vale is much easier mechanically in terms of simplicity (especially after the telegraphs are now visible nerf), but the dungeon itself is much more difficult in relation to that level because of both a stat limitation and skill limitation required players to perform a lot better than what is demanded -- the party themselves couldn't greatly exceed the difficulty of the dungeon. There's a lot less room for error. Aurum Vale is a 4-man party where you actually needed a tank, a healer, and DPS to do the dungeon.
A more mild example would be Bardam's Mettle (Lv 65 dungeon). That dungeon's mechanics are very straightforward. However, the mobs themselves can hit very hard and wipe the party if the tank and healer are not up to par with their mitigation and healing. There's room for error and mistakes, but it's one of those dungeons that really encourage all players to use all of their toolkit or they won't have a very easy time. This was especially the case with the gear spike in the past (ILV260 Shire gear and ILV274 HQ gear had a huge gap in defensive scaling in the past, but I'm not sure if it's still there in Endwalker after stat squish).
There's a huge reason why people say healers feel very superfluous in dungeons for that reason, and would rather have more engaging DPS gameplay when they are stuck pressing 1-1-1-1-1-1-1 in Endwalker -- and is magnified even more than Shadowbringers now that tanks no longer require as much attention on external healing with each passing expansion. Between big differences in maximum ILVL sync through gear and lots of skills, there's so much leeway that the difficulty feels like it has gotten lower, especially with how predictable mechanics are once you seen them enough times.