See discussion in Zulip about CustomSlackHttpClient here:
The slack-api-client is bound to okhttp 4.x and CustomSlackHttpClient seems to be an effort to make the component bin compatible with okhttp client 3.x. But the RequestBody.create method signature used is referencing the 4.x APIs instead of 3.x.
Perhaps we should override and force okhttp 3.x in the component pom.xml and do the necessary bin compat tweaks in CustomSlackHttpClient? I'm no expert on okhttp or the slack-api-client so not sure if this can work reliably.