No, they are not. Rooting involves modifying the phone software so that you can grant apps administrator access. Unlocking the bootloader means removing a restriction on which partitions of the phone can be flashed via fastboot. A bootloader unlock is commonly the first step towards rooting (unlock bootloader, flash a custom recovery, use the recovery to flash the changes needed to root the phone) but is not itself the same thing as rooting.
Do note that unlocking the bootloader usually factory resets the phone, as a security measure in case someone is trying to get past a privacy lock this way.
If you want a phone with an unlockable bootloader, many manufacturers allow unlocking anyway with no tricks needed. On my Pixel 2 for example you just enable the Developer Options menu (same as on any Android), then in there there's an option to allow "OEM unlocking". Select that and you can use fastboot to unlock it - no manipulation of the software required. The HTCs I used previously could be unlocked just by getting a token from HTCdev.com and using that to authorize the unlocking. There are other manufacturers who do this, I'm just naming those I know best. But beware: even if the manufacturer supports unlocking, some service providers block this from phones they sell. What are confusingly called "unlocked" phones (which here means "not locked to a service provider", nothing at all to do with the bootloader) are more likely to be bootloader-unlockable, but you need to check for the particular phone - there is no general rule here, it depends on manufacturer, sometimes on country, sometimes on model, and if sold through a service provider it depends on which. If you are buying second hand it's important to know where the phone was bought from originally, since a network unlock won't make e.g. a Verizon phone bootloader-unlockable (and the seller may not know anything about this, or even that there are 2 meanings to "unlocked").
Of course if nobody is doing development for a phone then there won't be a custom recovery for it, so I'd always check that as well.
Now if you mean "what's the latest Android version for which those shonky one-click rooting apps work?" that's an entirely different question, and nothing to do with bootloader unlocking. I avoid those things, so can't give a definitive answer, but most stopped working with Android 6. In any event, even if someone says "this app works with Android 6/7/8/whatever" that only tells you that it works with that particular phone and that particular software build (not Android version, since a security update can cut off a root method without changing the Android version). So the correct question is in general not "what's the latest Android version that this tool works with?" but "does this tool work with phone X, and if so does it work with all software versions?".
Sorry that's not a clear and simple answer, but the question of which phones can be rooted generally isn't.
