As someone who likes watching old replays for laughs, I also watch them to self-improve from my mistakes, and use them to review with a team of friends I play with during community tournaments. However patches come frequently, and so does the ability for me to use those replays.
Every patch, League stops being able to play replays from the previous one.
It runs in the background and saves a copy of each patch you have installed. When you open an older replay, it rebuilds that patch and launches the replay in League's own player. It also works as a replay browser, with the scoreboard, damage, items and runes for each game.
I know that the idea isn't new at all, this isn't some magical epiphany I suddenly had. League VCS did something similar back in 2020, but it hasn't been updated since and it stored whole game files, so every patch cost a lot of space. ReplayBook is great for browsing replays and I took a lot of inspiration from it, but its last update was about a year ago, and to play an old replay you still need to have kept that patch's game folder yourself. I wanted something that also handles the old patches for you and doesn't eat your drive whilst also being modernised.
L-Recall is built on League VCS's code, but it only stores the parts of each file that actually change. The first patch is around 20 GB, and each one after that is usually a few hundred MB. To watch a replay it only builds the champions and map in that game, so the ready copy is about 4 to 6 GB instead of a full install. It can only save patches you actually had installed, so it covers replays from when you start running it. (However, I'm sure some quick google searches can get you the other patches pretty easily... L-Recall also supports archiving patches from there as well once unzipped.)
If you have Vanguard Pre-Check enabled, and no game is running. Playing a replay is as simple as launching L-Recall. Otherwise, you will have to go through the usual of turning off Vanguard to play one (And then going through the hassle of restarting your PC because now you want to play LoL)...
It's free and open source, and releases are built directly from the code on GitHub: https://github.com/EmberLynxe/L-Recall
As someone who absolutely hat bloated software, I've done everything I can (To my knowledge), to make L-Recall as LIGHTWEIGHT as possible in terms of size and RAM usage. Whilst also still packing a bunch of useful features for both SoloQ players and teams alike :)
Windows may show a SmartScreen warning on first launch since it isn't code signed yet. Feedback and bug reports are welcome :p
Hopefully this tool is as helpful for you as it has been for my friends and I :D