Of course, it will depend very much on what, exactly, do you use. Symantec (Norton) announced continued support for XP when XP left. I imagine they might do the same with Win 7. Many other things continued to work. However, one by one, I found I cound NOT update a lot of software. This was especially true of TAX software. I guess the people who made tax software decided they didn't want the risk of running the older OS on their hands. Same with other financial software.
What I wound up doing was just to give up and go to Best Buy and get a $500 computer with a TB HDD, 12 GB ram, I5 CPU. I paid extra to upgrade Win 10 to PRO and also a new copy of OFFICE. Just decided to bite the bullet and move on. But, if you really want to stay with Win 7, here is what I think you'll see.
1. No more updates (without some hack). You may risk some security protection afforded by new OS patching. On the other hand, things written for specific OSes will move on or already have moved on to Win 10. Bad actors don't want to wast time writing stuff for old OSes that hardly anyone uses (included LINUX). If I want to send out ONE piece of malware that is going to have the biggest impact, it won't be designed for older OSes or less common OSes either.
2) If you stay on Win 7, you will start to see software and software updates that will NOT work on Win 7. I already mentioned financial and tax software, as examples.
3) Hardware -- A note that, if your current system was upgraded from Win 95, WIN NT, WIN Xp to Win 7, there may be a limit if you do decide to go with Win 10. Not every processor (e.g., older Pentiums) is supported on Win 10 so evaluate exactly how old your system really is. In addition, finding DRIVERS for older hardware can be problematic. My netbook came with XP and can handle a "sample version" of Win 7 but NOTHING else in terms of drivers is available. So, hardware-wise, it was easier to make the move to buy a new system, then trying to upgrade XP all the way to Win 10 and go through VISTA, Win 7, Win 8,....
3) LINUX, MAC and other alternatives. There is nothing wrong with going to LINUX or MAC. Just remember that those platforms are NOT immune to malware either. We had this arguement once on another forum and someone ended the conversation by pointing out that 8 of the top 10 vulnerabilities that month were LINUX issues. Or was that Apple?? I forget it was a few years ago. They pointed to the NIST website list of vulnerabilities for the month.
The point is that the support for programs went really quickly for the software I was using. I had less than a year to move on.