The list may not be exhaustive and there are probably more prop firms that block API access.
To work around this issue, you can use the non-API mode of the copier which is available on MatchTrader and DXtrade slave copier apps using the local version of Heron Copier.
Miscellaneous
List of Prop Firms with Blocked API Access
API (Application Programming Interface) is a channel that allows the copier to communicate with the trading platform.
Heron Copier (and most other copier apps) use the API to copy trades.
When the API is blocked, the copier (and all other copier apps) will not be able to connect to the trading platform.
Here is a list of prop firms that are known to block API access: