Windows 10 1507 (including 2015 LTSB, based on 1507) and 1511 are the only "safe" Windows 10 builds. Anything 1607 or later is compromised. Some people may tolerate the discomfort of later Windows 10 builds (and they vary in level of discomfort, ie some may find 1709/1803 tolerable, but 1903/1909 no good, and vice versa), but they are not "safe" for the most sensitive of users.
@tfouto is right. 1511 media refresh April 2016 (R3 as @tfouto refers to it) is the latest Windows 10 ISO you can download which does not contain the extra eyestrain component we attribute to 1607 and later. However, it does not receive security updates. If you want up to date security patches, then the only option is Windows 10 2015 LTSB. As explained before in this forum, we cannot give detailed instructions on activating Windows 10 2015 LTSB. If you are not as concerned about security (to be honest, I don't know squat about this and don't know either way how risky it is to run an unsupported version of Windows), then you can try Windows 10 build 1511, which is easily attainable from any website which allows you to download ISOs from Microsoft (such as TechBench). Personally, I would suggest seeing if 1511 is comfortable, if it is, and you are concerned about security, then do your research into 1507 LTSB (Windows 10 2015 LTSB).
I don't mean to discount people who have comfortable post-1511 setups, but I am adamant that for the most sensitive of users, with damaged eyes, 1607+ is not optimal, and can be an unexplored factor which leaves strain in place. I was unwavering in my believe for so long that OS-level eyestrain could not, and should not, be as potent as monitor or GPU-induced eyestrain. It's a reasonable expectation that you should be able to use modern Windows without trouble. I didn't want to take a second thought at it, it seemed unbelievable, frankly, that MS could introduce changes which cause eyestrain without experiencing mass outcry. Unfortunately, I was wrong, and it can be the difference between a comfortable setup and one that is completely unusable. It really sucks.
Special thanks to @KM for advancing 2015 LTSB as a solution for modern Windows 10 eyestrain. I have modern laptops which would be completely unusable except for his elucidation of this unconventional approach. Also thanks to @Gurm who brought to our attention the "composition layers" that Microsoft implemented post 1511 (or at the very least laid the back end for in 1607, producing symptoms).