To address your specific situation (Garen with Stridebreaker and Abyssal Mask) there are two questions that need to be answered.
Does Abyssal Mask's Unmake passive trigger off of immobilizing effects from items?
Yes. To test this, I jumped into the practice tool as Garen, spawned an enemy target dummy, and bought both Everfrost and Abyssal Mask. When rooting the enemy dummy with the centre of Everfrost's active it also received a debuff from Abyssal Mask which reads "This unit takes amplified damage".
Does a slow count as an immobilizing effect?
No. To test this, I simply hit the enemy target dummy with the edge of Everfrost's active effect rather than the centre, which slows instead of roots. In this situation, there was no debuff from Abyssal Mask.