The method MongoDB\BSON\UTCDateTime::toDateTime() returns a mutable DateTime instance. The community pushes for using ImmutableDateTime as much as possible.

• https://www.reddit.com/r/PHP/comments/w7v4pb/deprecating_the_mutable_datetime_class/ 
• https://blog.devgenius.io/stop-using-datetime-in-php-66df3d731875
• https://www.nikolaposa.in.rs/blog/2019/07/01/stop-using-datetime/

Currently, to get a DateTimeImmutable object from an UTCDateTime, developers have to do that:

$date = new UTCDateTime();
$dateTime = ImmutableDateTime::fromMutable($date->toDateTime());

Proposed implementations:

Add a method:

$dateTime = $date->toDateTimeImmutable();

Add a argument:

$dateTime = $date->toDateTime(mutable: false);

We can change the default value of mutable in 2.0 to return an immutable object by default. This change can be announced by deprecating not-passing an argument to the method.