Made me do a quick count from my web site logs. (I don;t currently track W7, since it is still possible to be viable, although not at the consumer level.)
As of 2020, I have at least
80 customers using XP SP2, XP SP3, or Server 2003
30 using Vista, Server 2008, or Server 2012
Not a large percentage, but for the data they are responsible for, should be zero. These are customers who are not going out of business (at least willingly) for 2020. Given my experience, the numbers will halve each year, at best.
Programming wise, provided one does not use "out there" Windows API calls, there is no programming need to prevent use with XP SP2 or later. Other factors can come into play, such as my preferred installation software vendor has dropped support for XP, which will eventually mean my software will not be able to install on XP either.
Like when DOS died, I will eventually have to force people to make a decision, by preventing use on certain OS's. (We only provide support and only test with current OS, and strongly suggest using only current OS.) It has only been about a year since someone asked about a DOS version, even though we stopped creating them in the last century...
Going through something similar now, with installation CD's (caused by the Wayfair ruling, and the low number of those actually needing a CD). The reality of current payroll processing is an internet available computer is needed, making a CD not needed, although some claim - despite web site logs showing differently - they cannot download because they have no internet access where they are.
As an old timer once told me (back when I was not the old timer), "Only worry about the 80%. If you try to handle everything the last 20% ask for, you will muck things up for the 80%." I would add those last 20% are the ones who keep you up at night, and the ones which could be costing money instead of earning money. The ~110 count are not even a tenth of a percent (thankfully), so while I absolutely appreciate their business, when I have to force them to move up or along, it will not (much) make me lose sleep.
If I have a point (It is first half of January, my mind is like yours must be April 15 or whatever your deadline dates are), is some will always be pound foolish, and some will get away with it. But, as good stewards of our customers, we must be penny wise.