What kind of complexities would make it so difficult to keep the XP11 version available for sale on the website? That doesn't make sense to me. I don't know a lot about hosting a site that sells downloadable products, but it can't be that difficult to leave it up on the store for people to buy and download. I mean, there's nothing else you really have to do (as far as I know) except keep it available in the store than just let it sit there for sale (with a "no longer supported" disclaimer) and keep the "master" copy of the aircraft on the site (or wherever you keep your downloads) available for people to download it. What else do you have to do that wasn't already done really?
There's an FSX/P3D aircraft developer that was named Milviz (now called Blackbird Simulations), they made a lot of high quality payware. About a year ago, they decided to drop out of the FSX/P3D market and focus on MSFS and then they did something very awesome for the FSX/P3D community (yes, they still exist), they re-released all 25 of there FSX/P3D aircraft as freeware and made them all available for anyone to download on their own website which they host themselves: Blackbird Simulations - Freebies (blackbirdsims.com)
If they can make 25 legacy aircraft available for download for free on their site, why can't you just keep one single legacy XP11 aircraft available (for sale)? Can it be that difficult to do?
As for still wanting to use XP11, I am certainly not unwilling to deal with XP12 because I own it already, I just don't use it much rn. Though it is an improvement over XP11, It's not nearly as impressive as I had hoped it would be and not much better than my heavily modified version of XP11, most of the time I find myself firing up XP11 instead. Also XP12 is barely even out of beta now. For XP11 I have many aircraft that are not and likely will not be updated for XP12 that I still enjoy flying, I have my Orbx TE scenery (most of which is not yet XP12 compatible) and loads of other types of scenery as well as many other kinds of add-ons, in fact, I have XP11 looking nearly as good as XP12 does and in some ways even better looking.
I'm not going to give up XP11 and the several hundred dollars' worth of add-ons I own that goes with it just to use XP12 which has very limited amount of content compared to XP11 rn and it will likely take a very long time for developers to do their thing and make old add-ons fully compatible (many converted XP11 to XP12 aircraft and other add-ons still have plenty of bugs & other issues to work out as well as the newly released content) as well as make more new add-ons. There's still a lot of upsides to still using XP11 for me. It is going to be a very, very long time before I switch to using XP12 on a regular basis, probably at least a few years or so, in fact I'm thinking about uninstalling it because I use it so little and it's taking up so much storage just sitting there.
I'm not buying the IXEG 737 if I can only get it for XP12, it's not worth it to me. I'm also not going to buy the new LES DC3 for the same reasons.