"This disk is unreadable by this computer"

HWTech wrote:

recent versions of macOS (Monterey & Ventura) may have trouble accessing really old media formats

Yes, Apple dropped support for quite a few old audio & video codecs upon the release of Catalina. That has caused problems for users who had used RealPlayer and other old audio apps ; and who had used various old video codecs in older versions of iMovie and other video editing apps.

Here is Apple's comprehensive list of the current & discontinuned codecs. (And although it's titled re Final Cut Pro, it is really the macOS system, not just Final Cut Pro): About incompatible media in Final Cut Pro - Apple Support

macOS can "read" virtually all common media formats. The limitations are generally 1) media that has defects and 2) limitations of the media device(s). If you have a CD/DVD drive or burner, check the specs on what kinds of media & formats it supports. Also check your media in another device to see if it can be read elsewhere.

DISC / discs can have different combinations of file-systems / formats:

HFS+ , PC (Joliet) , DVD (UDF) , ISO9660 , Joliet , UDF , HFS Standard / Joliet (only) , Joliet 103 characters (only) , etc etc etc etc.

To make the DISC compatible/recognizable/usable with macOS, you need to select

atleast HFS+ or HFS or UDF.

But to make the DISC compatible / recognizable / usable with other OS (operating-system) too,

you need & can MIX multiple formats in it , so that its usable+recognizable in multiple different OS.

so, better is to select & mix : HFS + Joliet + UDF,

or HFS + Joliet.

So your DVD / CD / Blue-Ray, etc burning / authoring software, needs to support various formats, & also support mixing them.

These pages showing what DISC & DRIVE & Network-Drive are supported by macOS:

File system formats available in Disk Utility on Mac - Apple Support

File System Details

you may also see my earlier this post's relevant portions.

I have seen reports that recent versions of macOS (Monterey & Ventura) may have trouble accessing really old media formats especially those used back in the PPC days, but it is hard to say whether this is because macOS no longer supports the really old formats or because of other issues causing the data to have become corrupted. I haven't seen enough credible information or details to say for sure. Of course this does assume the content is stored in a common format as mentioned by @MartinR.

You may want to try using a Linux system to access data on really old media, even old .dmg or .sparsebundle archives, although the archives take a little bit of extra work to access (as long as they are not encrypted) or use an older version of macOS to attempt to access older media (macOS 10.10 maybe?).

Burned CD/DVDs do degrade over time, so hopefully the user did not store their only copy of the data on CD/DVD.

Patdiddy wrote:

old CD, DVD, or various other forms of media

Exactly what various forms of media are you talking about?

If these are data CDs or DVDs, the media itself should be readable unless the media has gone bad or the CD/DVD drive isn't working for some reason. If the media are readable but the contents (files) cannot be read, then the question is exactly what are those files? (docs, spreadsheets, video files, etc)?

If these are playable audio CDs or video DVDs then, again, the media should be readable unless the media has gone bad or the the CD/DVD drive isn't working for some reason or is not capable of reading the disk.

If these are flash drives, how are they formatted?