The linked article in the original article relies for this assertion on Steven F. Huefner, "The Neglected Value of the Legislative Privilege in State Legislatures", 45 WM. & MARY L. REV. 221, 221 (2003). This article provides a state by state summary in an appendix starting at page 308 of that volume of the law review. See also pages 235-237 summarizing that data.
The states with no state constitutional speech and debate provision as of 2003 were: California, Florida, Iowa, Mississippi, Nevada, North Carolina and South Carolina. But, all of those states protect some of the things protected by such a constitutional provision either as a matter of case law or a state statute, so the distinction is really mostly one of style rather than substance and is mostly due to historical accident. The situation in those seven states is summarized in footnote 54 at pages 237-238 of the 2003 article:
The seven states are California, Florida, Iowa, Mississippi, Nevada,
North Carolina, and South Carolina. All but two of these states- North
Carolina and Florida- have a constitutional provision privileging its
state legislators from certain types of arrest or civil process during
the time the legislature is in session. Florida once had both a Speech
or Debate provision and an arrest provision in its Constitution, but
it has not had either provision since 1868. See Girardeau v. State,
403 So. 2d 513, 515 n.3 (Fla. Dist. Ct. App. 1981). Some interpreters
occasionally have used these arrest provisions also to support the
recognition of a common-law privilege of free legislative debate.
See, e.g., 1979-80 Op. Att'y Gen. Iowa 173, available at 1979 Iowa AG LEXIS 101 (using constitutional arrest privilege and limited
statutory Speech or Debate privilege to derive broad common-law
legislative privilege) [hereinafter Op. Att'y Gen. Iowa]. Such a
privilege, however, likely exists even in states without a
constitutional arrest clause. For example, the Florida Supreme Court
in dicta has strongly signaled its readiness to recognize a
legislative privilege as a matter of common-law in appropriate cases.
See Hauser v. Urchisin, 231 So. 2d 6 (Fla. 1970); see also Girardeau, 403 So. 2d at 516-17. In California, state courts
apparently have recognized a common-law legislative privilege for
state legislators, following the U.S. Supreme Court in Tenney v.
Brandhove. See Allen v. Superior Court, 340 P.2d 1030, 1034 (Cal.
Dist. Ct. App. 1959). But cf Hancock v. Burns, 323 P.2d 456,461
(Cal. Dist. Ct. App. 1958) (noting that inflicting bodily injury at a
hearing would not be privileged). Although Mississippi does not yet
appear to have expressly recognized a legislative privilege for state
legislators, it was one of the earliest jurisdictions to recognize a
common-law privilege for local legislators. See Jones v. Loving, 55
Miss. 109, 109 (1877). North Carolina also has recognized a common-law
legislative immunity for local legislators. See Stephenson v. Town of
Garner, 524 S.E.2d 608, 612-13 (N.C. Ct. App. 2000); Vereen v.
Holden, 468 S.E.2d 471, 473-74 (N.C. Ct. App. 1996). Meanwhile, North
Carolina and Iowa have statutory privileges. The North Carolina
statutory provision closely tracks the federal and typical state
constitutional provisions, stating that state legislators "shall have
freedom of speech and debate ... and shall not be liable to
impeachment or question, in any court or place out of the General
Assembly, for words therein spoken." N.C. GEN. STAT. § 120-9 (2001).
In contrast, Iowa's statutory provision expressly provides only that
state legislators "shall not be held for slander or libel in any court
for words used in any speech or debate in either house or at any
session of a standing committee." IOWA CODE § 2.17 (2002). The Iowa
attorney general has opined, however, that at common law state
legislators should receive the same broad immunity as provided by the
U.S. Constitution's Speech or Debate Clause. See Op. Att'y Gen.
Iowa, supra. Some states with constitutional legislative immunity
provisions also have statutory immunity provisions. See, e.g., MICH.
COMP. LAWS ANN. § 4.551 (West 1994). The District of Columbia also has
a statutory provision granting its legislative council a privilege
analogous to that provided by the federal Speech or Debate Clause. See
D.C. CODE ANN. § 1-301.42 (2001).