Do minipci 802.11ac cards (not minipci-e) exixst, to be installed in older routerboard equipped with minipci slot ?
(i.e. RB411, RB433 etc.)

RB400 series???....

And for sure you want run on it RouterOS v7....

With the price of one rare 802.11ac module for miniPCI slot (if exist....), you can buy directly one RB911-5HacD
and, if i not remember bad, has holes on board on same position...

Well, I have many of them (various 411, 333, 600, 800)
It would be fine to have them on AC mode
Unless AC performances are cut down by rb performance itself....

Of course it will... that board is half the speed of what a RB2011 was, and even that I already replaced.
There comes a time to say goodbye, I'm afraid.

Forget about RB411 or RB333, they are too weak for anything like this.
But RB600 and especially RB800 with miniPCIe slot still have plenty of power for 802.11ac traffic, especially if you don't need routing and just want AP.
As for supported radios, easiest is to list kernel modules in routeros-xxx.npk with 7zip and you get:
AR5211 AR5212 AR5416 AR9300 AR9888 QCA9888 QCA9984 WIL6210
Then check on wikidevi and basically only QCA9888 (2x2) and QCA9984 (4x4) are radio chipsets you want and only ones that will actually work with Mikrotik.
When you click on "X devices" link in the "Adapters" column on WikiDevi, you will get list of adapters/cards with that chip.
WLE650V5 with QCA9888 can be had used for about $12, that's not bad.
QCA9984 cards are more exotic and cost much more...

I wonder if someone has tried to replace the miniPCIe card used for 2 GHz in a RB4011 (desktop version) and if it could then run wifiwave2 on two bands?

Unless there is some specific limitation in wifi2 package, I don't see any reason why this would not work. You probably could even use mining pci-e riser and run multiple cards off that slot... at least I tried it some time ago with RB800 and all cards were detected and usable...

If it works, I think MikroTik should add a suitable card to their product line and offer it for free or at a discount to owners of the RB4011...
(as a compensation for not implementing concurrent operation of wifi and wifiwave2 drivers)

Unless there is some specific limitation in wifi2 package, I don't see any reason why this would not work. You probably could even use mining pci-e riser and run multiple cards off that slot... at least I tried it some time ago with RB800 and all cards were detected and usable...

On RB800 Which card did you try? What was the RouterOS version?

It's been like 3 years, so sorry I can't give you exact HW details. But I had some PCIe risers to play with so first I tried miniPCIe to single PCIe slot riser with Intel ethernet card in it. It showed in PCI resources, but no driver for it in PPC ROS. Next I tried some D-Link 802.11n Atheros based (AR9300) PCIe card and it showed and ran as wifi interface. Next I used one of mining boards with 4 slots that have it's own PLX chip. Again it showed in PCI resources and later I plugged in the D-Link as well as some 3 other random PCIe cards I had and they all showed in PCI resources list.
I was using external PC PSU for the risers and RB800 power for all of this.

After quick look at ebay "PCI-E 1X to 4X" search looks like the board with PLX.and miniPCI riser was one that looks like "Mini PCIe to PCI Express 16X Riser" with blue miniPci board with cutout for the cable. Just if you buy any of these and plan on connecting different risers together, ALWAYS buzz the PCIe pins with multimeter because there is NO STANDARD when it comes to sending PCIe over USB and they can be wired differently and you don't ever want to power up something like that before thorough testing (one riser could route 12V into PCIe bus on the incompatible one... so BEWARE!!!).

As for used ROS, 3 years ago it was clearly 6.4something.

I own few 600 and 800 too
The problem is to find AC minipci cards that aren't minipciE you can find plenty of those...

That's the advantage of RB800, it does have both mini-PCI and also one mini-PCIe slot you can use for these experiments...

Yes, miniPCI 802.11ac cards that are compatible with older routerboards equipped with miniPCI slots, such as RB411 and RB433, do exist.