Package sttill used? Is there a way to know if a package is still used ? (I tried with RedGate SQL Search and I found those in SERV-XXX and Maintenance Plans in various Job Steps, but for those in Data Collector I got nothing)
Nope. Unless you explicitly enabled logging within each and every SSIS package, there is no guaranteed way to know if it's being run. 

You can look through SQL Agent job steps, check for xp_cmdshell calls that either use dtexec.exe or call batch scripts which then use dtexec.exe and then discover some clever sod wrote a windows service that uses the SSIS object model to run them. Or it's the unscheduled process that is only run when Accounting says the books are closed at month/quarter/year end and only then is it run.