I bought an oil extractor from O'reilly's Auto Parts to try on my 2019 Softail Slim (FLS) today and have to say it did get most of the oil out of the reservoir. Since this was my first time using it -- I felt obligated to validate its performance.
I was able to drain a few tablespoons of oil from the drain plug underneath the bike. While I can be a little OCD about stuff like this -- the amount that was left wasn't concerning.
At one point, out of morbid curiosity, I removed the hand pump from the top of the unit and attached the hose for my shop-vac to generate the pressure needed to pull the oil out of the reservoir. It didn't really seem to make a noticeable difference, so I went back to factory settings.
If you're not aware -- the oil drain plug for the 2017 Harley-Davidson Softail Slim (FLS) is located on the same side of the bike as the oil tank ('right' side if you're sitting on the bike). It can be removed with either a 5/8" SAE socket or a 1/4" SAE hex key. It's been recommended to me on multiple occasions to actually ride the bike for at least a half-hour prior to changing the oil to sufficiently warm it up and circulate the oil through the engine completely. Allowing any air-cooled engine to idle is discouraged. It needs to be in the wind.
In addition, I took a small sample of the used Amsoil 5w20 that came out of the bike and sent it out to Blackstone Laboratories to have it analyzed. I'll post my results over on Bob Is The Oil Guy and have the hooligans over there take a look at them.