I got to wondering, "how many organizations actually still use old browsers"?
So, does your organization still use old browsers, if so which browser version and why?
I'd suspect that there are lots of smaller companies out there who are still using Windows XP (IE 8).
Don't expect a company to change its IT policies just to be able to use your software.
(I work in the enterprise software space, and my employer's customers include lots of big companies. We only recently dropped support for IE 8.)
In SME world, there is less of everything to coordinate, and that means there is more flexibility and to some extent the standard software used in the organisation tends to be updated more frequently. It's still likely that anywhere big enough to have dedicated IT staff also has controlled software deployment, but at least that's likely to be IE11, reasonably recent Safari on macOS, or possibly something like Firefox LTS or a centrally managed Chrome deployment.
I want to make something usable by many, but I'm just not going down the path of supporting everything back to IE8 - the resource is not there to do that development.
If you want to go for enterprise sales then you need quite a different approach to most other development work, not just in browser compatibility but in how you do things like marketing and sales and technical support. It's tough for any small business to do this, but if you can pull it off without the overheads of larger businesses then the profits can be substantial. Of course, you do have to support whatever platform your big customers are using. That could be IE11 these days; requests for compatibility with older versions of IE do seem to be fading out, I suspect because support for pre-7 Windows versions from Microsoft is now very expensive. But realistically, if you're a small outfit supplying to enterprises, you're probably working on almost a case-by-case basis anyway, and you just have to find out what each customer is using.
If you don't have the resources to handle such a demanding customer, or you just don't enjoy coding for the older browsers and want to work on projects that can use the new toys, I'd suggest aiming for a totally different type of customer in the first place. If you're writing web sites or apps aimed at home users rather than businesses then in most Western countries the very clear trend is to run Chrome on desktop/laptop and either Chrome or Safari on phones/tablets depending on whether it's Android or Apple. Safari doesn't update to add new features rapidly like the evergreen browsers, but Apple do tend to push out updates along with new versions of iOS, and in any case people tend to upgrade their mobile devices much more frequently than their desktops/laptops so the lag is less of a problem in practice than it was with IE.
Small businesses are somewhere in between, and the basic rule is that the smaller the business, the more likely it is to be using individual PCs and mobile devices rather than some organisation-wide standard, and so the more likely it is that people will be running very recent browsers. As soon as an organisation is big enough to have dedicated IT staff and central management of IT systems, which I'd guess starts around the 25-50 person scale for a lot of businesses, you're going to see much greater standardisation and therefore somewhat slower updates.
So pragmatically, if you're aiming at businesses then you probably need to support IE11 at least, but going beyond that is probably into enterprise sales territory. For a consumer market, it's almost all about Chrome and Safari these days IME, but you should probably try to support Safari as it was a couple of years ago as a minimum standard and ideally a bit further back for those who have older Apple devices that they can't/won't upgrade to the latest OS and browser.