What is the old-style encoding mentioned in the Objective-C Property Type Encoding documentation?
t<encoding> Specifies the type using old-style encoding.
Also what is
The property is eligible for garbage collection.
mean. Is it related to the Garbage Collector that Objective-C once had or has to do with ARC retain counts?
I think they made slight changes to the type encoding format a long time ago (possibly when they went to ObjC 2.0 and added properties, back in the MacOS 10.5 time frame, or even earlier). I'm not sure I've seen the 't' actually used; it may be something used by the gcc compiler and not clang. The 'T' value is the type encoding which is used today.
For the second question, yes that is for the obsolete garbage collector.