If you are accessing your files through dolphin on your Linux device this change has no effect on you. In that case Synology is just sharing files and it doesn’t know or care what kind of files they are.
This change is mostly for people who were using the Synology videos app to stream videos. I assume Plex is much more common on Synology and I don’t believe anything changed with Plex’s h265 support.
If you were using the built in Synology videos app and have objections to Plex give Jellyfin a try. It should handle h265 and doesn’t require a purchase like Plex does to unlock features like mobile apps.
Linux isn’t dropping any codecs and should be able to handle almost any media you throw at it. Codec support depends on what app you are using, and most Linux apps use ffmpeg to do that decoding. As far as I know Debian hasn’t dropped support for h265, but even if they did you could always compile your own ffmpeg libraries with it re-enabled.
How can I most easily search my NAS for files needing the removed codecs
The mediainfo command is one of the easiest ways to do this on the command line. It can tell you what video/audio codecs are used in a file.
With Linux and Synology DSM both dropping codecs, I am considering just taking the storage hit to convert to h.264 or another format. What would you recommend?
To answer this you need to know the least common denominator of supported codecs on everything you want to play back on. If you are only worried about playing this back on your Linux machine with your 1080s then you fully support h265 already and you should not convert anything. Any conversion between codecs is lossy so it is best to leave them as they are or else you will lose quality.
If you have other hardware that can’t support h265, h264 is probably the next best. Almost any hardware in the last 15 years should easily handle h264.
When it comes to thumbnails for a remote filesystem like this are they generated and stored on my PC or will the PC save them to the folder on the NAS where other programs could use them.
Yes they are generated locally, and Dolphin stores them in ~/.cache/thumbnails on your local system.
If you are just looking to repurpose an old device for around the house use and it won’t ever be leaving your home network, then the simplest method is to set a static IP address on the device and leave the default gateway empty. That will prevent it from reaching anything other than the local subnet.
If you have multiple subnets that the device needs to access you will need a proper firewall. Make sure that the device has a DHCP reservation or a static IP and then block outgoing traffic to the WAN from that IP while still allowing traffic to your local subnets.
If it is a phone who knows what that modem might be doing if there isn’t a hardware switch for it. You can’t expect much privacy when that modem is active. But like the other poster mentiond a private DNS server that only has records from your local services would at least prevent apps from reaching out as long as they aren’t smart enough to fall back to an IP address if DNS fails.
A VPN for your phone with firewall rules on your router that prevent your VPN clients from reaching the WAN would hopefully prevent any sort of fallback like that.