Whelp, guess i'm done with youtube then...
Most ads under 30 seconds are not skipable, it depends on the whims of the person running the ad (the company being advertised). I like running music channels that are like 8h of X ambient, those have random 5-45 minute ads and those are typically skippable but are for the dumbest products like medication or financial literacy classes / seminars.
All that is water under the bridge though for one simple reason...the company can make ads not skipable.
I've had several "Pride" themed ads from Youtube that weren't skipable at all. If youtube is running "Oh you adblock, no videos!" right now, then it is only a matter of time before they say, "No Video unless you watch all the ads without skipping!" or add some pandora level gay shit like X ad skips per hour.