browserl.ist | browsers supported by a browserslist query | Browser Plugin library

You can do npx browserslist to list all the browsers included in your browserslist query.

Gulp is not needed to run that command but if you wanted to, you could create a Gulp task to run it in your terminal.

When, if ever, will IE 9, 10, and 11 no longer appear on lists of the last three versions of web browsers?

The way sites like that are built, probably only when IE is dropped from the lists altogether. FWIW, caniuse.com already shows only version 11 by default — presumably its developer was smart enough not to hardcode the limit to 3 for every browser. So does the ES6 compatibility table.

Furthermore when will it become commonplace to no longer expend the effort to support them?

It already is commonplace to no longer support Internet Explorer 9 and 10. Microsoft ended support for all versions of IE older than 11 on January 12, 2016, and most of the top sites have followed suit since then. More than two years have passed, and I wouldn't be surprised if mostly only legacy sites support old IE anymore (which is fine: if it ain't broke, don't fix it). Mine is just about the only non-legacy site I'm aware of that actively supports Internet Explorer 8, and I'm expending that effort myself solely for personal reasons (tl;dr: I'm sentimental and I think it's funny to push so hard against the tide for something so insignificant).

End of support for Internet Explorer 11 is much harder to telegraph. Aside from it being a secondary browser on most versions of Windows 10 (i.e. notwithstanding Windows 10 Enterprise LTSB which does not include Edge at all, and custom OOBEs that set IE11 as default or something), the other pain point here is Windows 7, which goes EOL in January 2020. (Most Windows 8.1 users have already upgraded to Windows 10.)

As stated in my answer to this question, security and compatibility updates are still shipping for IE11, and will continue to do so for the lifetime of the version of Windows it's installed on. Presumably this means IE11 on Windows 7 will stop receiving updates after January 2020, IE11 on Windows 8.1 after January 2023, and IE11 on Windows 10 will continue receiving updates for the foreseeable future — provided the customer keeps Windows 10 itself up to date (specifically, on either of the last two major updates).

For most consumer-facing sites, it's likely that efforts to stop supporting IE11 will end as Windows 7 approaches EOL in much the same way old IE bled out in 2016. But this is theoretical at best, and if you are primarily building sites for enterprise, as mentioned in my linked answer you're probably going to have to continue supporting it for at least a few more years until Edge becomes an enterprise mainstay.