A quick update on this - After a long discussion with UiPath Marketplace team, they have approved my custom activity code in .Net4.6 to be published on Marketplace.
I am assuming that the UiPath Dev team is currently working on this System.Activities for .net Core or higher stable version.
Thanks
Yoichi
ashokkarale
Anil_G
Ajay_Mishra
AkshaySandhu
jast1631
Gowthaman_Subramanian
Damai_Revanth
ppr
mayank_26