TFAYD
Technically we have trip dropping through Sked+, but there needs to be sufficient reserves in the appropriate bucket to cover the entire trip, in base, and maybe even system wide, to actually drop into open time. Since we are chronically understaffed, there are never enough reserves to cover your trip, so you can never drop.
There is the ability to post and pray that someone else wants your trip, but if it's anything other than a high paying local or a good standup, fuhgeddaboudit.
Occasionally you can trade with someone, or split the trip up into more desirable pieces to get someone to pick it up. Cash helps too.
You can also exchange a pairing you don't want with one in company open time, but company constraints on number of days, number of reserves on the days you want to get rid of, and other unknown line constraints usually result in Failure
When all else fails, platinum days.
In summary, we have the option to drop, but usually that translates to the inability to drop