magento2: REST API multiselect attribute BC break between 2.1.8 and 2.3.0

Preconditions (*)

1. Magento 2.4-develop

Steps to reproduce (*)

1. Have a multiselect attribute
2. Update the product data through the REST API and pass an array of IDs as a value for this multiselect attribute

Expected result (*)

1. Attribute is updated (as it worked in 2.1.8)

Actual result (*)

1. Error message is returned:

{
    "message": "Error occurred during \"custom_attributes\" processing. Attribute \"equipment\" has invalid value. The \"array\" value's type is invalid. The \"string\" type was expected. Verify and try again.",
    "trace": null
}

Workaround

Implode the ids and send them as a string. But it makes your code incompatible with Magento 2.1.8 as it expects an array of ids.

I guess that it is some kind of BC break, because Magento 2.1.8 works in a completely opposite way - it requires an array of IDs and rejects string with a message: {"message":"Error occurred during \"custom_attributes\" processing. Attribute \"equipment\" has invalid value. Invalid type for value: \"string\". Expected Type: \"string[]\"."}

Related issue: #6120