Legacy custom bots has been deprecated on March 31st, 2025.

[znz]

ログを確認すると、
https://github.com/rosylilly/ruboty-slack_rtm/blob/cc6c191bd2e4c32840c7dd434669fb0dae23d2ff/lib/ruboty/adapters/slack_rtm.rb#L117 で nil を渡していてエラーになっていたので、手元で https://slack.com/api/rtm.connect の返り値を確認したところ、以下のようなエラーになっていました。

%  SLACK_TOKEN=xoxb-ひみつ irb
irb(main):001> require 'net/http'
=> true
irb(main):002> require 'uri'
=> false
irb(main):003> response = Net::HTTP.post_form(URI.parse('https://slack.com/api/rtm.connect'), token: ENV['SLACK_TOKEN'])
=> #<Net::HTTPOK 200 OK readbody=true>
irb(main):004> body = JSON.parse(response.body)
=>
{"ok" => false,
...
irb(main):005> body
=>
{"ok" => false,
 "error" => "legacy_custom_bots_deprecated",
 "context" =>
  "Legacy custom bots has been deprecated on March 31st, 2025. Please create a new app using our updated methods. More info can be found in our changelog: https://api.slack.com/changelog/2024-09-legacy-custom-bots-classic-apps-deprecation"}

[znz]

https://github.com/tomoasleep/ruboty-slack_events に移行すれば良さそう。