I just did PADI OW last month and I could still do 300m MSF.
I'd have to look up when it was taken out but I think it was like a year ago. Maybe your instructor hasn't been reading his updates...

R..
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I just did PADI OW last month and I could still do 300m MSF.
1. the watermanship assessment for these courses [Scuba diver and Open Water diver] you must conduct the ‘200 metre continuous surface swim and a 10-minute swim/float without using any swim aids’ prior to the first open water dive. Note that you cannot use the ‘300 metre swim with mask, snorkel and fin and a 10-minute swim/float without using any swim aids’ as an alternative to this.
The scuba instructor shall limit the number of students on open water dives so that either the scuba instructor or a safety diver is able to establish physical contact with every student at any time during the open water dive’
I am guessing the swim test is only in the pool and not also in open water?