Where the ad was bought, not where it ran
Google defines utm_source_platform as "the platform responsible for directing traffic to a given Analytics property", such as a buying platform that sets budgets and targeting, or a platform that manages organic traffic data. Its own examples are Search Ads 360 and Display & Video 360. The value names the tool someone used to run the campaign.
utm_source answers a different question. Google's examples for it are google, newsletter4 and billboard, the place the visitor came from. One campaign bought in Display & Video 360 can send traffic from many sources, and the platform value stays the same across all of them.
How GA4 files it
GA4 has a built-in Source platform dimension. Its listed values include Google Ads, SA360, Shopping Free Listings, and Manual for traffic that is not from Google's own media. utm_source_platform feeds a separate set: Manual source platform, Session manual source platform and First user manual source platform. Look there, not under Source platform, when you want to see what your tag recorded.
Why Google asks for it
On its UTM help page, Google says that if you set one UTM parameter you should set all the relevant ones, and it names utm_source_platform in that list with source, medium, campaign and utm_id. Leave it off and the manual source platform dimension reads (not set) for those visits. For a team running each campaign through one platform, the gap costs little. For a team buying the same campaign through several platforms, this is the tag built to tell them apart.
Stripping it, and spelling it
Removing it changes nothing for the visitor. It names a platform, not a person, so there is no privacy reason to cut it from a link you share. For the marketer, the visit loses that one dimension. If you set it, pick one spelling and keep it. Google treats values as case sensitive, and a value with spaces such as Search Ads 360 becomes Search%20Ads%20360 in the URL. Build the full set in the UTM builder.