It is possible to delete each run separately.
So first get all Runs (filtered or not filtered).
Workflow Runs List
with runId you can delete this runs runs.
Workflow Run Delete
You need a similar Http-Request to to delete a run:
DELETE https://management.azure.com/subscriptions/{subscriptionId}/resourceGroups/{resourceGroupName}/providers/Microsoft.Logic/workflows/{workflowName}/runs/{runName}?api-version=2016-06-01
For the autorization yout need a bearer token, I am not sure but I think you get it on powershell with that commands:
az connect
az account get-access-token
Then you got s.th. like this:
Authorization: Bearer eyJ0eXAiOiJKV1QiLCJhbGciOiJSUzI1NiIsIng1dCI6Im5PbzNaRHJPRFhFSzFqS1doWHNsSFJfS1hFZyIsImtpZCI6Im5PbzNaRHJPRFhFSzFqS1doWHNsSFJfS1hFZyJ9.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.QGJdK4Z-s-JUreJtS1gn6BbGvTPeLTJ-PJuZKYasSlZaSuJyt6GIosW-8-bBR8dDb3tFvL50J68cAHQeSmOR8TOnLknt6ERNnSR773LlL8bmm527m29jNzpQGb51r41PM5QxcwHS3mKlMyD894lwhnV-Z6dcXlLc3zmXcDmVRKjMKPFV6c6RBJUlKvaQK-IHMpmqe3SagvCI5r0ETNDus5qK_nd0zSo-b1nsquZ8SlNJsZFnC5b39C1GdsHZdU6dis61vkg_Gpy1tNJ4JjtbqgQHxnpcBYVbEe7OacejMK2rV_Z0sr2Ij00O91YBuI8f-soE4Ot_SsAbhQ82ZhkLHA
So if parsing your run history, you could delete with that command runs that e.g. older than 1 day.
The only problem is now for me. It worked perfect in the browser on the microsoft site, on powershell I got the problem to send a valid post or the token is invalid.
If i get this working will post it here.