According to https://dev.twitch.tv/docs/api/migration the Search Streams option is no longer available but we can
Use “Search Channels” to match channel name or title.
This is simply invalid because if you put some existing title into query the channel having it is not returned. The search only works on channel names but does not work on descriptions/titles.
This seems to be a documentation bug.
BTW - maybe some additional flag to search in channel titles should be introduced for such case?
Another problem: thumbnail_url description is all wrong:
Thumbnail URL of the stream. All image URLs have variable width and height. You can replace {width} and {height} with any values to get that size image.
This is not a thumbnail of the stream but channel logo and does not have variable width and height.
XQC example: default 300x300 works 100x100 does not work (403).
That a user profile image. Not a stream thumbnail!.
This is bug reported Channel Search does not search titles · Issue #545 · twitchdev/issues · GitHub
Precisely, and that is what is returned from channel search. 
If the channel is not live.
then no stream thumbnail is returned
It would be great to have it documented this way, I guess I will file a bug.
BTW Thanks for github link, somehow I have only searched forum for those issue and not the github page.
There is also a uservoice: Stream title searching – Twitch UserVoice
Testing against the endpoint looks like you have found a weird.
The API says “thumbnail_url” the documentation say “thumbnail_url” but it’s the profile picture being returned not a stream thumbnail.
Live/Not Live streamers returned by search the thumbnail_url is always the profile picture.
twitch api get 'search/channels?query=cohhcarnage'
{
"data": [
{
"broadcaster_language": "en",
"broadcaster_login": "cohhcarnage",
"display_name": "CohhCarnage",
"game_id": "490100",
"game_name": "LOST ARK",
"id": "26610234",
"is_live": true,
"started_at": "2022-02-16T12:24:34Z",
"tag_ids": [
"c2542d6d-cd10-4532-919b-3d19f30a768b",
"6ea6bca4-4712-4ab9-a906-e3336a9d8039",
"96b6073f-450d-4248-8ed4-988e28f3f759",
"54b544af-2d0e-4d17-a653-78e3f6fc1fc8",
"eaba0ad7-c4e1-4878-b37f-01308dbb65c8",
"1277e554-f8c9-4795-a475-27418d7bd111"
],
"thumbnail_url": "https://static-cdn.jtvnw.net/jtv_user_pictures/e3a58ce0-46ea-4558-8bb0-7a967c1d621c-profile_image-300x300.png",
"title": "[!DROPS] - !MERCH until 2/27! - Lost Ark (Ladon / Artillerist) - HORIZON: FORBIDDEN WEST SOON - !LAThoughts - YouTube.com/CohhDaily - !PC"
},
{
"broadcaster_language": "en",
"broadcaster_login": "cohhcarnage1",
"display_name": "CohhCarnage1",
"game_id": "0",
"game_name": "",
"id": "80339555",
"is_live": false,
"started_at": "",
"tag_ids": [],
"thumbnail_url": "https://static-cdn.jtvnw.net/user-default-pictures-uv/ead5c8b2-a4c9-4724-b1dd-9f00b46cbd3d-profile_image-300x300.png",
"title": ""
},
So something is wrong with search
I don’t know if this endpoint is supposed to return stream thumbnails or not (I never have a use for search myself)
Or the documentation. The mechanism to return profile picture for non-live channels is OK, but the live channels should probably have thumbnail_url.
I think the best solution would be to return both thumbnail_url and (new property) profile_picture/channel_logo (thumbnail_url would be empty if channel is not live or would return thumbnail from last stream).
What do you think? Do you want me to file a bug?
Sure documentation says it’s a Thumbnail URL of the stream but it never is.
So a bug of some description.
As to what the endpoint is supposed to do I don’t know