Hebrews 8:13 is

> "By speaking of a new covenant, He has made the first one obsolete;
> and what is obsolete and aging will soon disappear." (Berean Study Bible)

It is important to clarify what the term 'obsolete' here means. As the lines make clear, the first covenant 'will soon disappear'. This means it still exists. So whatever 'obsolete' here means, it does not mean the Old Covenant does not exist at the time of the writing of Hebrews, but rather the opposite.

παλαιούμενον (palaioumenon) Strong's 3822: To make old, declare obsolete; pass: I grow old, become obsolete. From palaios; to make worn out, or declare obsolete.

The primary sense of 'obsolete' here is to make old. Indeed, Hebrews 1:11 uses the same word as παλαιωθήσονται (palaiōthēsontai), typically translated there as 'grow old' or 'wear out'.

So the question is at what point did the Mosaic Covenant grow old or wear out (but still exist)? I don't think there's a specific moment, but there are various points that are important. Jesus' birth, his baptism with the Holy Spirit and the beginning of his ministry, his Transfiguration, the Last Supper, his crucifixion, his resurrection, Pentecost, and the Ascension are all key points where the New Covenant is coming into being or being moved forward and the old one 'becoming old' - the word is sufficiently vague to include all these points.

What we can say is that the Old Covenant, although it has become old or 'worn out', is still in effect (for some) at the point of the writing of Hebrews.