Short answer: you can't do it natively. A matplotlib.text.Text is a single artist — matplotlib has no per-line visibility, styling, or handles for the individual lines of a Text. set_visible, set_alpha, etc. always apply to the whole string.

The workaround that keeps the string untouched is to clip the artist so only the region below the first line is rendered:

import matplotlib.pyplot as plt
from matplotlib.transforms import Bbox

fig, ax = plt.subplots()
t = ax.text(0.5, 0.5, "first line\nsecond line", va="top")

fig.canvas.draw()  # a renderer is needed to measure extents

# Full bbox of the text, in display (pixel) coordinates
bbox = t.get_window_extent()

# Measure one line's height with a throwaway probe with identical font properties
probe = ax.text(*t.get_position(), "Ag",
                fontsize=t.get_fontsize(), fontfamily=t.get_fontfamily(),
                va=t.get_va(), ha=t.get_ha(), rotation=t.get_rotation(),
                transform=t.get_transform())
line_h = probe.get_window_extent().height
probe.remove()

# Lines stack downward from the anchor, so hide the top of the bbox:
# keep only the bottom (bbox.height - one line stride) of the text.
n_lines = t.get_text().count("\n") + 1
stride = (bbox.height - line_h) / (n_lines - 1) if n_lines > 1 else 0
keep = bbox.height - stride

t.set_clip_on(True)
t.set_clip_box(Bbox([[bbox.x0, bbox.y0], [bbox.x1, bbox.y0 + keep]]))

Caveats to be aware of:

• The clip box is specified in display coordinates (which is why the extents from get_window_extent() work directly), and it is static — it won't follow redraws (Stack Overflow). If the figure is resized, panned, or saved with a different DPI, the clip will be wrong, so recompute it on a draw event (fig.canvas.mpl_connect("draw_event", ...)) or right before savefig after a fig.canvas.draw().
• The stride/line_h measurement assumes uniform line height (no per-line font changes, \n-separated lines).
• For a general "hide lines k..m" you'd clip the same way but keep the sub-rectangle between two multiples of stride from the bottom.
• An advanced alternative is Text._get_layout(renderer), which is what matplotlib internally uses to lay out lines — but it's private API and unstable across versions.

If the constraint "don't alter the text" is only about visual output (not the actual string), the pragmatic options are simpler: replace the first line with spaces of the same length (" " * len(line)) to preserve the block layout, or split the text into one Text artist per line and set_visible(False) on the first.