Leszek Podhorodecki in "Dzieje Ukrainy" ("History of Ukraine") writes:
"(...) At the turns of the 5th and the 6th centuries the Slavs, living until that time at the Dniester River, attacked the borders of the Byzantine Empire. The whole reign of Justin (518-527) and that of Justinian (527-557) were filled with combats against the Slavs pushing south across the Danube. They were especially active in period 545-557, because at that time they started to settle en masse in conquered territories [south of the Danube]. Only the incursion of the Avars into the Black Sea steppe and the lands along the Danube [years 561 - 569], hampered - for some time - the Slavic migration. After victorious wars against [some of] Slavic tribes, the Avars penetrated into the Pannonian Basin, and established their realm there. (...)" - from page 18
And the Dniester River is here:
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These are Slavic tribes ancestral to Slavic Macedonians (red lines show borders of the region of Macedonia):
Some of those tribes became Hellenized, but Slavs were still the most numerous ethnic group in Macedonia:
Ethnic groups in Macedonia before WW1
After WW1 at least 618,200 immigrant Anatolian Greeks settled in Macedonia (patterns of settlement of Anatolian Greeks throughout Macedonia and other regions of Greece can be seen in maps posted below, 1st map shows the proportion of Anatolian immigrants to local populations in each region):
Proportion of immigrants from Asia Minor (by region):
And settlements of immigrants from Asia Minor (dots):
Comparing the size of Greek population of southern Macedonia before WW1 (236,800) and the number of Anatolian Greek immigrants after WW1 (618,200) shows that Anatolian Greeks outnumbered Macedonian Greeks 3 to 1 in that area, and they significantly changed the ethnic makeup of that territory.
Simultaneously with the influx of at least 618,200 Anatolian Greeks, at least 130,010 of local Slavic inhabitants (out of 370,371 - rounded to 370,400 in my chart above) were deported from southern Macedonia towards northern regions of what later became Yugoslavia and to western Bulgaria. Deportations of Turks also took place. Jews (most of whom lived in the city of Salonica) were later wiped out in WW2, further decreasing the number of non-Greek minorities.
Possibly much more Slavic-speakers than 130,010 left that region after WW1 - but not all of them necessarily in organized deportations.
My next finding is that the exodus after WW1 was not the last one - because during and after the Greek Civil War (1946 - 1949) over 100,000 more Slavs from Greek Macedonia had to emigrate to Yugoslavia and Bulgaria, further decreasing the size of Slavic minority in Greece.
Yet some Slavic minority in southern (Greek) Macedonia still exists - for example according to the report of Human Rights Watch "Denying Ethnic Identity. The Macedonians of Greece", in 1992 over 65% of inhabitants of the district of Florina (Φλώρινα) described themselves as Slavic Macedonians.