Plugin/Paper Version here:

https://www.curseforge.com/minecraft/bukkit-plugins/offline-furnaces

How it works:

In vanilla, when you left items processing in furnaces and the chunk gets unloaded, it stops, no ticks running on the furnace. So when you come back to the same chunk, it continues the process.

Everytime the mod detects there is something burning on a furnace (it can be normal, blast, smelter) it will keep that chunk loaded until the last item is finished. To avoid performance issues this is done ONLY when processing items (actively burning). Fuel is not a trigger.

Features: 

• Works with all types of vanilla furnaces. 
• Server-side only.
• Works with furnaces connected to hoppers.
• To keep a good performance, if there are more than 1 furnaces running on the same chunk, the mod will keep active the entire chunk, and will wait for the last one to finish its items.
• Added the command "/of debug" (only available to OPs) so you can check how many active chunks are loaded (by the mod) and also how many furnaces within those chunks are running, with exact coordinates.

Installation: 

• First, MAKE SURE to disable this parameter in your server.properties:
  pause-when-empty-seconds=-1
  If you dont disable it, it will stop working due to Fabric pausing the server ticks when no players are online.
• Drop the .jar file into your mods folder. 
• Restart.

Be aware: 

• This mod WILL BECOME AN ISSUE if you are using crazy-giant furnace farms on your server. Since the main event here is keeping the chunk loaded, the more chunks you load, the less the performance.
• I won't be backporting this mod to older versions. I tested it in all 1.21.x Fabric versions and it's properly working. However, you can try to use it in 1.20.x but do not expect to work on all previous versions.
• NO forge ports.

Bye!