What firmware version were you using before the update? The Kobo firmware has been using that
www.msftncsi.com as part of the check for connectivity for several years. Before that, it used an Apple url. The change to
www.msftncsi.com was at a time that Kobo added support for using hot spots.
As to what URL to use, there would be an argument for using a Kobo owned URL and server, but people would argue about what would happen if Kobo went under. Using this MS site makes a reasonable amount of sense as it is being used for the same purpose by at least one other operating system. And the resources (servers, bandwidth, etc) backing it make it unlikely that it will be unavailable.
And if your adblocking system has recently decided to block this URL, I would suggest you change to another. Deciding to block this this domain or URL as a source for ads would make me question if the people running the system knew what they were doing.