If you don't want to read on, for it does become a bit technical, but found a furnace-like block that you feel Heater should propagate heat to but doesn't, then I invite you to check if an issue regarding said block already exists or, if it doesn't, to open a new one. If you open a new one, I ask you to specify the mod name and possibly a source-code link, the furnace-like block or blocks, and the Minecraft version. Note that if a mod isn't open source, I can do close to nothing to make it compatible.

That said, Heater should be automatically interoperable with any 3rd party mod's block whose entity extends the AbstractFurnaceBlockEntity (as from Mojang mappings) class.

This is because every entity extending such a class has the litTime and litDuration (as from Mojang mappings) fields, which this mod interacts with to propagate heat.

However, some mods (like Heater, paradoxically) don't extend said class for the furnace-like blocks they add but still have two fields that work the same as litTime and litDuration.

To make such blocks interoperable, you can create a data pack containing one or more furnace adapters, as in the following examples.

<datapack>.zip
├── data
│   └── <datapack>
│       └── heater
│           └── adapters
│               └── furnace
│                   └── <block_entity_type_1>.json (content example below)
│                   └── <block_entity_type_2>.json
│                   └── ...
├── pack.mcmeta
└── pack.png (optional)

// custom_furnace.json
{
    // The id of the 3rd party mod's furnace-like block's entity' type
    "type": "example_mod:custom_furnace_entity_type",
    // The name of the field corresponding in function to `litTime`
    "lit_time": "burnTime",
    // The name of the field corresponding in function to `litDuration`
    "lit_duration": "fuelTime"
}