There's a boatload of stuff from 1.5 to 4.0, which wouldn't work any longer. Everything you listed above, plus all the download enhancements with libcurl, the embedded files, even the console capabilities were expanded since then so you can locate and _CINP and such...
The issue that arises is like I mentioned though -- XP doesn't support the underlying libraries or function calls to make those things work. There's no curl in XP, so no downloading library or API to pull from. There's no calls to WinConsole until Vista+ as those functions weren't added to Windows until then. A lot of your underlying libraries simply don't exist in 1.5, so you wouldn't be able to run 4.0 code and compile in it and expect the same results. Sure, the same PRINT "Hello World" would work in both, but anything that came since.... even before Phoenix Edition existed, simply wouldn't work.
1.5 -- That's 3 *major* releases in the past. You can't expect anything from a 4.x version to have a chance in hell of working on a 1.x version.
If you're going to use XP, you're going to be limited to using an ancient version of QB64 and whatever it has packaged with it. We're talking pre-Phoenix Edition. Pre-Official. Pre-RC Cola... You're going back in time and running an OS which is over 20 years out of date, with a compiler and version of QB64 which is at least 5 years out of date. That's enough that I'd suggest keeping expectations low on what you can with it, compared to the modern version which we offer.
https://github.com/QB64Team/qb64/releases/tag/v1.5
(And I was wrong with the 5 year mark... It's only 4 and a half years out of date.
)
The issue that arises is like I mentioned though -- XP doesn't support the underlying libraries or function calls to make those things work. There's no curl in XP, so no downloading library or API to pull from. There's no calls to WinConsole until Vista+ as those functions weren't added to Windows until then. A lot of your underlying libraries simply don't exist in 1.5, so you wouldn't be able to run 4.0 code and compile in it and expect the same results. Sure, the same PRINT "Hello World" would work in both, but anything that came since.... even before Phoenix Edition existed, simply wouldn't work.
1.5 -- That's 3 *major* releases in the past. You can't expect anything from a 4.x version to have a chance in hell of working on a 1.x version.
If you're going to use XP, you're going to be limited to using an ancient version of QB64 and whatever it has packaged with it. We're talking pre-Phoenix Edition. Pre-Official. Pre-RC Cola... You're going back in time and running an OS which is over 20 years out of date, with a compiler and version of QB64 which is at least 5 years out of date. That's enough that I'd suggest keeping expectations low on what you can with it, compared to the modern version which we offer.
https://github.com/QB64Team/qb64/releases/tag/v1.5
(And I was wrong with the 5 year mark... It's only 4 and a half years out of date.
)