I literally can not think of anything software wise, especially FOSS as I use the command line a lot and those tools and concepts go back decades. Being a retro computer geek I can list a ton of old proprietary systems or software that I consider perfectly usable.

Oh wait, I just thought of one: RiscOS Open. The best OS for ARM besides Linux, all my Pi's run on it and it natively uses BBC BASIC, although not Free as in Freedom BBC BASIC, or even BASIC in general is a programming language that has a lot to offer.

Although not software I think the biggest thing I have in mind would be Optical Media. Most consider it obsolete, even against data tape, but I use it extensively precisely because it has features no other media possesses (ignoring LTO tape). Featurea such as many decades of longevity, cheapness (even today it's cheaper than equivalent sized flash media) and above all it's the only media that has read only properties.

SSD's, HDD's are not close to archival grade, only optical and tape (ignoring film and the ultimate archival media, vellum) are.

All my data that must be recovered at all costs is archived to BD-R, which in turn is backed up to LTO tape, which in turn is backed up into the cloud. Both the bd-r and LTO tape are written and finished days before the data has been uploaded to the cloud! Because my upload speed is 20Mb/s maximum the old SCSI LTO 4 drive writing to tape at 60MB/s wipes the floor with it, the bd-r records much slower than that but still is done in a fraction of the time.

Maybe if I'm ever able to get 1Gb upload bandwidth I'll use the cloud more, but at the moment it's running at a slower speed than my first 486 with it's 210MB HDD!

Edit: Ah! Wait, I forgot to mention Window Maker. I use Window Maker as my window manager. Works like a charm and hasn't changed in looks one bit since the 90's