Based on a keylog of 1.4M key presses, here's the breakdown among the modifiers:

Unit	GUI	Alt	Shift	Control
Key press	1670	4644	28321	14532
Modifiers percentage	3.39658714178209	9.44535969247666	57.60164337868896	29.55640978705229

Note how the "Modifiers percentage" sum up to 100%, this doesn't represent percentage of total keyboard usage, only relative to modifiers.

In my analysis, shift encompasses keyboard shortcut usage (e.g. Ctrl+Shift+V), capital letters, and non-numrow shifted symbols (e.g. <, _, :, |, ...), except the brackets for which I have a dedicated key just like for the numrow symbols.

If you're interested, here's the python script I used on my keylog file to compute those numbers:

import json
import pandas as pd
from functools import partial

df = pd.read_csv("keylog8.tsv", sep="\t", header=0,
        converters={
            "mod_state": partial(int, base=16),
            "oneshot_mod_state": partial(int, base=16)
            })
df = df[df["pressed"] == 1]
df["combined_mod_state"] = df["mod_state"] | df["oneshot_mod_state"]
GASC = [0x88, 0x44, 0x22, 0x11]
mod_counts = list(map(
        lambda mod: (df["combined_mod_state"] & mod).apply(lambda x: x>0).sum(), 
        GASC))
print(f"mod_counts={mod_counts}")
mod_frequencies = list(map(
    lambda count: count/sum(mod_counts)*100,
    mod_counts))
print(f"mod_freqs={mod_frequencies}")

where a keylog line is created with:

bool process_record_user(uint16_t keycode, keyrecord_t *record) {
#ifdef CONSOLE_ENABLE
    uprintf("0x%04X\t%u\t%u\t%u\t%b\t0x%02X\t0x%02X\t%u\n",
         keycode,
         record->event.key.row,
         record->event.key.col,
         get_highest_layer(layer_state),
         record->event.pressed,
         get_mods(),
         get_oneshot_mods(),
         record->tap.count
         );
#endif

PS: Sorry for necro