I've made a website that hosts backups of old Half-Life tools. Most of this is from Slackiller and a Gamefront mirror, but i'll add more stuff as needed. All files are hosted on Github.
https://samvanheer.github.io/Half-Life-Tools-Backup/
Credits to Shepard for making the HLEnhanced website, which i've used as a base for this.
Currently hosted are the Slackiller HL Programs page and all the Half-Life SDKs that i could find.
This includes SDKs 1.0, 2.0, 2.1, 2.2 and 2.3, including separate releases of files.
An interesting thing to note is that the early SDK releases required the signing of an NDA prior to allowing the creation of a mod. Mods also had to be hosted on an FTP server in order to be made visible to the Custom Game browser.
I'm currently going through the list of utilities on the Gamefront mirror (https://gamefront.online/files/listing/pub2/Half_Life/Tools_and_Utilities) to mirror anything useful.
I also took the various SDK source codes and made branches that show the evolution of the multiplayer and single player codebases:
Multiplayer codebase: https://github.com/SamVanheer/Half-Life-Tools-Backup/commits/hl1_src_mp
Singleplayer codebase: https://github.com/SamVanheer/Half-Life-Tools-Backup/commits/hl1_src_sp
Difference between the multiplayer and singleplayer codebases in 2.3: https://github.com/SamVanheer/Half-Life-Tools-Backup/commits/hl1_src_2.3_diff
I didn't catch most of the file removals (if any), just the removal of the "dedicated" directory in 2.4 (Github release).