An excellent question! It turns out that Google Sheets is much less resistant to constructing your own matrices than Excel is, so you can do away with the convoluted CHOOSE function. You can just replace CHOOSE({1;2},{0,…,28},{0,…,28}) with {0,…,28;0,…,28}

Or if you want the formula in full:

=DATE(year,3,21)+
INDEX(
{0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,28;
0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,27,28},
IF(MOD(year,19)>=11,2,1),
MOD(15+INT(year/100)-INT(INT(year/100)/4)-INT((13+8*INT(year/100))/25)-11*MOD(year,19),30)
+1)+8-WEEKDAY(
DATE(year,3,21)+
INDEX(
{0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,28;
0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,27,28},
IF(MOD(year,19)>=11,2,1),
MOD(15+INT(year/100)-INT(INT(year/100)/4)-INT((13+8*INT(year/100))/25)-11*MOD(year,19),30)
+1),1)